Ion implantation is a standard technique for introducing conductivity-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the substrate material and are embedded into the crystalline lattice of the substrate material to form a region of desired conductivity.
Solar cells provide pollution-free, equal-access energy using a free natural resource. Due to environmental concerns and rising energy costs, solar cells, which may be composed of silicon substrates, are becoming more globally important. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells may require doping to improve efficiency. The dopant may be, for example, arsenic, phosphorus, or boron. FIG. 1A is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the p-n junction is on the back side of the solar cell. As shown in FIG. 1B, the doping pattern includes alternating p-type and n-type dopant regions in this particular embodiment. The p+ emitter 203 and the n+ back surface field 204 are appropriately doped. This doping may enable the junction in the IBC solar cell to function or have increased efficiency.
Some solar cells, such as IBC solar cells, require that different regions of the solar cell be p-type and others n-type. It may be difficult to align these various regions without overlap or error. For example, the p+ emitter 203 and n+ back surface field 204 in FIG. 1B must be doped. If overlap between the p-type regions 203 and the n-type regions 204 exists, counterdoping may occur. Any overlap or misalignment also may affect the function of the solar cell. For solar cells that require multiple implants, particularly those with small structure or implant region dimensions, the alignment requirements can limit the use of a shadow mask or proximity mask. In particular, as shown in FIG. 1B, an IBC solar cell requires alternating lines doped with, for example, B and P. Therefore, any shadow mask or proximity mask for the B implant has narrow, long apertures that are carefully aligned to the small features that were implanted with P using a different proximity mask or shadow mask.
Accordingly, there is a need in the art for an improved method of implanting a solar cell and, more particularly, a method of using a shadow mask to create a soft mask that can be used for subsequent implants.